Type 1 diabetes mellitus (T1DM) is an autoimmune disease, thought to be caused by a T cell mediated destruction of the islets of Langerhans. We have recently shown that treatment of patients with new onset T1DM with a non-FcR binding anti-CD3 monoclonal antibody, hOKT3gamma1(Ala-Ala) arrests the progression of disease for at least 1 year after onset. Our original studies included patients within 6 weeks after diagnosis, but the drug may, have similar beneficial effects after that time. The mechanistic studies, funded by the ACE program showed evidence for T cell activation in vivo and that clinical responders could be differentiated from non-responders by a relative increase in the ntunber of CD8+ T cells. In this proposed Project, we will build on these observations and use samples and studies of patients in a new clinical trial to determine the effects of anti--CD3 mAb treatment on T1DM. In the first Aim we will carry out a randomized, placebo controlled trial of hOKT3gamma1(Ala-Ala) in patients with T1DM of disease duration longer than 6 weeks, with a protocol previously approved by the ACE. The treatment will involve two courses of the drug and a 3 year follow-up after diagnosis. The study will answer whether the treatment effect is limited to the period immediately after diagnosis, and, by quantitative and qualitative dynamic testing of insulin production, whether there is evidence of beta cell regeneration. In Aim 2, we will characterize the CD8+ T cell population after treatment to determine the functional role of these cells and specifically test the hypothesis that they have a regulatory function for antigen-specific CD4+ T cells. Finally, recent data indicates that beta cell mass is the end result of an equilibrium between destruction and regeneration. Our finding that some patients have an increase in insulin secretory function is consistent with the notion that beta cell mass may increase after mAb treatment. Therefore, we will study regeneration of beta cells in patients (with insulin responses to the mixed meal test) and in the NOD mouse. These clinical and mechanistic study results may substantiate the use of anti-CD3 antibody after clinical onset of disease, with the goal of allowing beta cell regeneration once tolerance to the disease has been established.